A patch covered in tiny microneedles can regrow hair in mice, scientists report. The patch could lead to treatment for hair loss in people, say its creators, a team led by Zhen Gu of the University of California, Los Angeles, and Liang Zhang of the Chinese Academy of Sciences. Gu, Zhang, and coworkers created the microneedles using a hydrogel containing keratin, the protein that makes up hair. They reasoned that this material would be both biocompatible and biodegradable. They loaded the microneedles with UK 5099, a small molecule that’s known to promote hair growth, and exosomes from a certain type of stem cell that can activate hair follicles. The microneedles are almost invisible once they’ve been applied to the skin and their hyaluronic acid backing has been removed. In tests with shaved mice, the microneedle system regrew hair in 6 days with just two treatments (ACS Nano 2019, DOI: 10.1021/acsnano.8b09573). Mice that got topical treatments of UK 5099 and exosome injections regrew hair more slowly. Untreated mice didn’t regrow hair at all. Gu thinks the microneedle patch brings its therapeutic cargo directly into the hair follicle stem cells’ niche, making it possible to steadily deliver low doses of the therapy where it’s most useful.